As one example of an apparatus for forming thin films (e.g., silicon nitride (SiN) films) on substrates such as semiconductor wafers (hereinafter referred to as “wafers”) or the like, there is known a batch-type vertical heat treatment apparatus which performs a film forming process collectively on a plurality of wafers within a vertical quartz-made reaction tube. As a specific film forming method using this apparatus, there is employed, e.g., a so-called ALD (Atomic Layer Deposition) method in which a silicon-containing source gas and a reaction gas (e.g., an ammonia (NH3) gas) for nitriding the source gas are alternately supplied plural times. Silicon nitride films are formed not only on surfaces of the wafers but also on outer surfaces of gas injectors for supplying the respective gases, an inner wall surface of the reaction tube, or the like.
However, the silicon nitride films formed on the gas injectors or the reaction tube are very large in internal stress of the film and are greatly different in a thermal expansion rate and a thermal contraction rate from quartz of which the reaction tube is made. Thus, along with heating and cooling of the reaction tube, the silicon nitride films are easily separated from the surface of the gas injectors or the reaction tube. For that reason, if a process for forming the silicon nitride films is repeated, the silicon nitride films separated from the surface of the gas injectors or the reaction tube become particles and adhere to the wafers. This leads to a reduction of throughput.
There is known a technique in which adhesion of particles to wafers is prevented by heating and cooling a reaction container after silicon nitride films are formed. In this technique, however, an electric current larger than an electric current used during an ordinary process is supplied to a heater for heating the interior of the reaction container. Therefore, the heater is easily deteriorated (the lifespan of the heater is shortened). Further, in this case, when cooling the reaction container, a cooling gas having an extremely low temperature of 0 degrees C. or so is blown from the outside of the reaction container. For that reason, when restoring an internal temperature of the reaction container to a temperature for a process to be subsequently performed, the internal temperature of the reaction container is difficult to be stabilized.
There are known a technique of using two boats, a technique of evacuating the interior of a reaction chamber when unloading a boat, and a technique of discharging particles by supplying a purge gas into a reaction chamber at a large flow rate when loading or unloading wafers. In these cases, however, it cannot be said that the techniques are capable of sufficiently suppressing adhesion of particles to wafers.